The proposed studies will dissect the surface antigenic markers of a cell of profound biologic and clinical significance. Elucidation of histocompatibility HL-A antigens will discern chemical factors affecting cryptic surface expression, a phenomenon rare in HL-A immunobiology, but common to tumor-specific and weak histocompatibility systems. Trophoblastic organ-specific antigens are of import not only for studies of auto-immune dyscrasias, but also in relation to tumor-immunity and as abortifacients. Analysis of tumor-specific common and distinctive surface markers may discern components of immunodiagnostic and immunotherapeutic significance to gestational trophoblastic neoplasia. Because of the unique situation of a foreign genetic origin, trophoblastic neoplasms may posses strong antigenic differences from their hosts. This possibility has been well recognized but only indirectly approached in studies performed to date. The proposed work will elucidate the importance of maternal reactivity toward strong histocompatibility antigens in resistance toward trophoblastic malignancy. The unique advantages of the applicants' approach are (1) A large patient population, (2) The capacity to extract and purify paternal HL-A antigens, and (3) Sensitive immunoassay techniques to detect specific host immunocompetence. Studies of the immune reactivities of patients during the period of tenuous host-tumor balance following termination of a molar pregnancy may afford insight into the factors determining clinical tumor progression or regression.